Facing Phobos

Jun 29, 2003

Mars has two natural satellites, or moons, Phobos and Deimos. On June 1, 2003, the Mars Global Surveyor (MGS) spacecraft was slewed eastward to capture these views of the inner moon, Phobos, shortly before it set over the afternoon limb. Phobos orbits Mars about 3 times a day at a distance of about 6,000 km (3,728 mi). About 0.006 times the size of Earth’s Moon, Phobos is a potato-shaped object with dimensions approximately 27 by 22 by 18 kilometers (about 17 by 14 by 11 miles).

Mars Moon, Phobos, potato-shaped satellite, 2003Credit: MSS/JPL/NASA

The banner shown here is a color composite of four MGS Mars Orbiter Camera (MOC) wide angle images; the tiny whitish-gray dot at the upper left shows the location of Phobos. The inset views are MOC narrow angle images, showing pockmarked details on the surface of the tiny moon.

Mars Phobos crater, 1998Credit: MSS/JPL/NASA

Phobos is one of the darkest objects in the Solar System. Thus, four wide angle images were obtained to make the picture of Phobos over the martian limb: a pair of red and blue wide angle images was acquired for the limb, and a pair of separate images were required to see Phobos.

The wide angle images illustrate the fact that Phobos is mostly colorless (dark gray); the faint orange/red hue in the wide angle picture is a combination of slight differences in the focal lengths of the blue and red cameras and the orange/red illumination provided by reflection of sunlight off Mars. To a person standing on Phobos, the red planet would fill most of the sky.

The high resolution image (right) was taken at the same time as the wide angle views. MGS was about 9,670 kilometers (6,010 miles) from Phobos when the picture was taken. At this distance, the image resolution is about 36 meters (118 ft.) per pixel; the maximum dimension of Phobos as seen in this image (the diagonal from lower left to upper right) is just over 24 km (15 mi). This is the “trailing” hemisphere, the part of Phobos that faces opposite the direction that the moon orbits Mars. This is a part of Phobos that was not seen by MOC in 1998 (left), when MGS made several close flybys of the tiny moon.

The rows of grooves and aligned pits on Phobos are related to, and were probably caused by, a large meteor impact that occurred on the side of Phobos. That large crater, Stickney, was named for the maiden name of the wife of the astronomer that discovered Phobos and the other martian satellite, Deimos, in 1877, Asaph Hall.

Mars Global Surveyor has been orbiting the red planet since Sept. 12, 1997. The mission has examined the entire Mars surface and provided a wealth of information about the planet’s atmosphere and interior. A new batch of high resolution photos, taken between February and July 2002, were added online in April and they bring the total number of images in the online gallery to more than 123,800. The images are available from the Mars Orbiter Camera Gallery.

What’s Next

Twin NASA rovers and one European Space Agency probe are planned for Mars landing in late 2003 and early 2004. The first Mars Exploration Rover, or MER, will arrive at Mars on Jan. 4, 2004, the second, Jan. 25.

Plans call for each to operate for at least three months. These missions continue NASA’s quest to understand the role of water on Mars. “We will be using the rovers to find rocks and soils that could hold clues about wet environments of Mars’ past,” said Dr. Cathy Weitz, MER Rover program scientist at NASA Headquarters. “We’ll analyze the clues to assess whether those environments may have been conducive to life.”

“We see the twin rovers as stepping stones for the rest of the decade and to a future decade of Mars exploration that will ultimately provide the knowledge necessary for human exploration,” said Orlando Figueroa, director of the Mars Exploration Program at NASA Headquarters.

The observed atmospheric clarity is higher than was expected from Earth-based microwave measurements and Hubble Space Telescope observations.

Dust is confirmed as the dominant absorber of Solar radiation in Mars’ atmosphere, which has important consequences for the transport of energy in the atmosphere and its circulation. Frequent “dust devils” were found with an unmistakable temperature, wind and pressure signature, and morning turbulence; at least one may have contained dust (on Sol 62), suggesting that these gusts are a mechanism for mixing dust into the atmosphere.

Morning atmospheric obscurations are due to clouds, not ground fog; Viking could not distinguish between these two possibilities.

The weather was similar to the weather encountered by Viking 1; there were rapid pressure and temperature variations, downslope winds at night and light winds in general.

Temperatures were about 10 degrees warmer than those measured by Viking 1.

Diversity of albedos, or variations in the brightness of the Martian surface, was similar to other observations, but there was no evidence for the types of crystalline hematite or pyroxene absorption features detected in other locations on Mars.

The atmospheric experiment package recorded a temperature profile different than expected from microwave measurements and Hubble observations.

Rock size distribution was consistent with a flood-related deposit.

The moment of inertia of Mars was refined to a corresponding core radius of between 807 miles and 1,242 miles (1,300 and 2,000 kilometers).

The possible identification of rounded pebbles and cobbles on the ground, and sockets and pebbles in some rocks, suggests conglomerates that formed in running water, during a warmer past in which liquid water was stable.

The historical accounts of previous Mars expeditions are outlined in tabular inset, according to date, mission and results.